1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and an apparatus for manufacturing a semiconductor device and, more particularly, to a dicing step and dicing apparatus for forming semiconductor chips by dividing a semiconductor wafer.
2. Description of the Related Art
Conventionally, a dicing step in the semiconductor device manufacturing process is performed as shown in FIGS. 1A, 1B, and 1C. That is, as shown in FIGS. 1A and 1B, a semiconductor wafer 11 in which elements are formed is cut one way along dicing lines 12-1, 12-2, . . . , by using a diamond blade (rotating blade) 13. The wafer 11 is then rotated 90° and, as shown in FIG. 1C, diced again in a direction perpendicular to the former dicing direction. In this manner, the wafer 11 is divided into individual semiconductor chips 14-1, 14-2, 14-3, . . . .
The above dicing step can be performed by either a full-cut method by which the wafer 11 is completely cut, or a half-cut method by which the wafer 11 is diced to about ½ the thickness of the wafer 11 or to a depth with which the wafer 11 remains by about 30 μm.
The half-cut method requires a dividing operation after dicing; the wafer 11 is sandwiched between flexible films or the like and divided by applying an external force by rollers. When an adhesive sheet is adhered before dicing, the wafer 11 is divided via the sheet by applying an external force by rollers or the like.
In a die bonding step, each of the chips 14-1, 14-2, 14-3, . . . , of the divided wafer 11 is mounted on a lead frame. More specifically, the lower surface of the adhesive sheet of each of the chips 14-1, 14-2, 14-3, . . . , is pushed up by a pickup needle, and the needle is brought into direct contact with the back surface of each chip through the adhesive sheet. Each chip is further raised and removed from the adhesive sheet. The removed chip is conveyed as its upper surface is held by suction by a tool called a collet, and mounted on a die pad of the lead frame.
Subsequently, a wire bonding step is performed to electrically connect pads of the chips 14-1, 14-2, 14-3, . . . , to inner leads of the lead frame. When the chips are to be mounted on a TAB tape, a heated bonding tool is used to electrically connect pads of these chips to leads of the TAB tape.
After that, a packaging step is performed to encapsulate each chip in a resin or ceramic package, thereby completing a semiconductor device.
In the dicing step of the manufacturing method as described above, however, strain or chipping occurs on the side surface of each semiconductor chip owing to cutting streaks, and this lowers the bending strength of the chip. Therefore, if stress is applied which is produced by the pressure applied in the pickup step performed before the step of mounting the chip on the lead frame or TAB tape, or by the difference between the thermal expansion coefficients of the packaging material and chip, this stress concentrates to the strain or chipping, so the chip cracks from this strain or chipping.
Recently, to embed a semiconductor chip in a thin card-like package or the like, a manufacturing method is used by which when a semiconductor wafer is cut, the backside of the pattern formation surface (semiconductor element formation surface) of the wafer is thinned by grinding using a wheel or by polishing using free abrasive grains, and then the wafer is cut by dicing. Also, a technique called DBG (Dicing Before Grinding) is proposed to form thinner chips (e.g., Jpn. Pat. Appln. KOKAI Publication No. S61-112345). In this DBG, a cut (half cut) is formed to a predetermined depth from the element formation surface of a wafer, and the back surface of the wafer is ground to divide the individual chips and decrease the thickness at the same time.
By this technique, chipping on the back surface or on an edge between the side surface and back surface of a chip can be removed by polishing the back surface of a semiconductor wafer. However, strain or chipping caused by cutting streaks formed on the side surface of a semiconductor chip cannot be removed. This inevitably lowers the bending strength when the thickness of a chip is decreased. Consequently, it is impossible to completely solve the problem that semiconductor chips crack in the assembly steps or in the reliability test before they are packaged, and defective products are formed.
In recent years, therefore, a technique which cuts a semiconductor wafer by laser beam irradiation, instead of dicing using mechanical cutting as described above, is attracting attention (e.g., Jpn. Pat. Appln. KOKAI Publication No. 2001-144037). This cutting using laser beam irradiation can eliminate streaks or chipping by mechanical cutting. However, a laser beam requires high power, so the side surface of a chip is damaged or roughened by recrystallization after melting. This inevitably lowers the bending strength. Also, a material melted by laser beam irradiation scatters and contaminates the surface of a chip. Furthermore, when a semiconductor wafer is to be cut via an adhesive sheet, the wafer must be irradiated with a laser beam twice by changing the focusing position (in the direction of depth of the wafer) of the laser beam. This complicates the dicing step.